A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The lithographic apparatus comprises one or more objects that need to be positioned, e.g. a substrate table, a support structure to support a patterning device or an optical element. In order to accurately position the object, displacements and/or the position of the object must be accurately measured. Any error in the measurement of the displacements and/or position of the object may lead to the object being assumed to be in the wrong position or may lead to errors in positioning the object. It is desirable to accurately measure displacements and/or position of an object such that, for example, the object can be moved to a specific location.
Known measurement systems may include the use of an electromagnetic sensor. An electromagnetic sensor has a driving circuit configured to drive an electromagnet to generate an alternating magnetic field. If the electromagnetic sensor is close to the object the alternating magnetic field interacts with the object, and consequently the object affects the alternating magnetic field. Thus the presence of the object affects an electrical impedance parameter of the electromagnet in a manner that depends on the relative position of the object and the electromagnet. Thus location and/or movement of the object relative to the electromagnet can be detected as changes in the electrical impedance parameter. Thus the displacement and/or position of the object may be determined. However, errors in the distance measured by the electromagnetic sensor may be inherent in the measurement and it is desirable to reduce or prevent errors in the measurement of the displacement and/or position of the object.